Combustion devices

ABSTRACT

The invention relates to combustion devices wherein a flame stabilisation zone is formed in the wake of a body disposed in a swiftly moving gas stream. The invention provides a combustion device comprising a bluffbody disposed in a gas stream, the body having an aerodynamically smooth upstream-facing portion terminating downstream through a step change of section in a substantially flat downstream-facing surface extending transversely to the direction of flow of the stream, and means for influencing flow over the body to induce stable vortex formation adjacent said downstream facing surface. Preferably such means comprise suction means, suction being applied from the interior of the body in each region of the step change of section. Combustion devices according to the invention may include a flow divider extending downstream of the downstream facing surface to separate vortices originating at the step change of section.

United States Patent [191 Bryce et al.

[ June 18, 1974 COMBUSTION DEVICES [75] Inventors: William Dean Bryce,Farnham;

Peter George Street, Fleet, both of England [73] Assignee: The Secretaryof State for Defence in Her Britannic Majesty s Government of the UnitedKingdom of Great Britain and Northern Ireland, London, England [22]Filed: Oct. 31, 1972 [21] Appl. No.: 302,395

[30] Foreign Application Priority Data Nov. 1, 1971 Great Britain50635/71 [52] US. Cl. 431/350, 60/39.72 R

[51] Int. Cl. F02g 1/00 [58] Field of Search 60/39.72 R, 261; 431/350[56] References Cited UNITED STATES PATENTS 2,771,743 11/1956 Lovesey60/39.72 R

3,600,892 8/1971 Bryce 60/39.72 R

3,618,318 11/197] Bryce 60/39.72 R

Primary Examiner-Meyer Perlin Assistant Examiner-Larry I. SchwartzAttorney, Agent, or FirmCameron, Kerkam, Sutton, Stowell & Stowell [57]ABSTRACT The invention relates to combustion devices wherein a flamestabilisation zone is formed in the wake of a body disposed in a swiftlymoving gas stream.

The invention provides a combustion device comprising a bluff-bodydisposed in a gas stream, the body having an aerodynamically smoothupstream-facing portion terminating downstream through a step change ofsection in a substantially flat downstream-facing surface extendingtransversely to the direction of flow of the stream, and means forinfluencing flow over the body to induce stable vortex formationadjacent said downstream facing surface.

Preferably such means comprise suction means, suction being applied fromthe interior of the body in each region of the step change of section.

Combustion devices according to the invention may include a flow dividerextending downstream of the downstream facing surface to separatevortices originating at the step change of section.

4 Claims, 1 Drawing Figure PATENTEDJun 18 I974 COMBUSTION DEVICES Thepresent invention relates to combustion devices wherein a flamestabilisation zone is formed in the wake of a body disposed in a swiftlymoving gas stream.

In aircraft gas turbine jet propulsion engines, extra power can bedeveloped by burning additional fuel in an afterburner. It has long beenthe practice in such installations to use fixed baffles to producestable turbulent (or recirculation) zones in their wakes wherein flameswill maintain themselves. Such baffles offer undesirable resistance toflow during such time as afterbuming is not in operation. One of themore common forms of fixed baffle is a V-shaped gutter having its apexpointing upstream of the flow, the extreme simplicity of the arrangementtending to outweigh adverse considerations. By setting the arms to anincluded angle of the order of 45, or slightly less according to flowvelocity, a reasonable optimum between resistance and the provision ofan adequate recirculation zone can be obtained.

Nevertheless, baffle resistance can be critical in de- 7 termining thetotal amount of afterbuming which can be employed, and an after-bumersystem having a lower pressure loss when in the non-operating conditionis desirable, provided that the penalties arising out of otherconsiderations are not too severe. The present inventionis concernedwith means directed towards this end.

Various schemes have already been proposed with this end in view andmany of them employ aerodynamic means. One of the more common forms ofdevice comprises a body having a profile which normally gives rise tolow drag and from which flow is caused to separate when afterbuming isrequired by discharging air or other gas from the body across thedirection of main flow. It has also been proposed, for example in priorBrit. Pat. No. 1,2 l 8, l 22, to utilise a body normally giving rise tohigh drag with resultant turbulent wake and, when after-burning is notrequired, to influence the boundary layer of flow over the body so as toreduce the wake. The present invention is on similar lines but wakereduction is achieved in somewhat different manner.

A combustion device according to the invention comprises a memberdisposed within a gas flow duct so as to produce a flame stabilisationzone in a swiftly moving gas stream and having a bluff-bodyconfiguration including a smooth upstream-facing portion with stepchange of section to a substantially flat downstream-facin g surfaceextending transversely relative to the direction of flow through theduct, and means for influencing flow over the member to induce stablevortex formation adjacent said downstream-facing surface.

In a preferred form of the invention, suction is applied from theinterior of the member in the region of the step change of section.

An embodiment of the invention will now be de scribed by way of examplewith reference to the accompanying diagrammatic sole FIGURE which is anaxial section through an afterburner duct including a flame stabiliser.

The afterburner duct shown in the FIGURE comprises a cylindrical casing1 extending between a turbine outlet and a jet propulsion nozzle(neither of which is shown), the direction of gas flow being from leftto right as indicated by the arrows at the left. An annular tubularmember 2 mounted coaxially within the duct has an aerofoil leading-edgesurface 3 directed upstream relative to flow through the duct andterminates downstream in a flat surface 4 normal to the streamwise axisof the member, a step change of section occurring between the twosurfaces. Annular ori fices 5, 6 extend through the peripheral boundaryof the member 2 in the region of change of section between the surfaces3, 4.

Gases passing along the duct will flow around the curved surface 3 ofthe member 2 as far as the change of section. In the absence of anyexternal influence, flow will separate from the member 2 at the changeof section to form turbulent recirculation zones in its wake as shown inthe upper part of the drawing. Liquid or gaseous hydrocarbon fuelintroduced into the duct and ignited will burn in the recirculation zonewhere the turbulence will act to maintain a flame pattern.

When afterbuming is not required, the fuel supply is discontinued andsuction is applied to the interior of the member 2. The suction acts byway of the orifices 5, 6, as indicated by the arrows S in the lower partof the drawing, on flow passing over the member 2 to form vortices Vadjacent to the downstream facing surface 4. Where the amount of suctionapplied is sufficient to maintain rotation of the vortices at a velocitysimilar to that of the normal flow through the duct, entrainment of thelatter will occur causing it to be changed to a streamline stable flowpattern within a very short distance, with consequent drag reduction.

The precise mechanism whereby this effect is achieved is not yet fullyunderstood but it is believed that the initial application of suctioncauses diversion of flow boundary layer whereby a vortex is initiated,some of the vortex flow in turn being drawn off by the suction until astage is reached when the main flow suddenly changes direction, somewhatafter the manner of a bi-stable fluidic switch. The boundary layer offlow over the curved surface, instead of breaking away as previously istransferred mainly to the vortex flow and the vortex becomes stabilisedduring such time as suction is maintained, part of the boundary layerbeing drawn into the orifices 5, 6.

The orifices 5, 6 may be in the form of substantially continuous slotsextending around the member 2, or in the form of a series of shorterslots or they may be de fined by a row of holes.

In an embodiment of the invention there is provided a flow dividerextending along the duct from the downstream-facing surface 4, asindicated in dotted lines at 7 in the drawing, in order to keep thevortices separate and of the same order of magnitude since there may bea possibility of one entraining the other.

What we claim is:

l. A combustion device for installation in a gas flow duct to formtherein a flame stabilisation zone, said combustion device comprising amember having a bluff-body configuration including an aerodynamicallysmooth upstream-facing portion, said portion terminating downstreamthrough a step change of section of the member in a substantially flatdownstream facing surface extending transversely relative to thedirection of said flow, and suction means establishing low pressure inthe interior of the member to influence said gas flow over said member,a stable vortex formation adjacent said downstream-facing surface beinginduced thereby, said member having at least one opening in each regionof step change at opposite edge portions of said downstream-facingsurface wherethrough suction is applied.

2. A combustion device as claimed in claim 1 including a flow dividerextending downstream of said downstream-facing surface whereby vorticesoriginating at said step change of section at opposite edge portions ofsaid downstream-facing surface are separated.

occupying a plane normal to the direction of gas flow.

1. A combustion device for installation in a gas flow duct to formtherein a flame stabilisation zone, said combustion device comprising amember having a bluff-body configuration including an aerodynamicallysmooth upstream-facing portion, said portion terminating downstrEamthrough a step change of section of the member in a substantially flatdownstream facing surface extending transversely relative to thedirection of said flow, and suction means establishing low pressure inthe interior of the member to influence said gas flow over said member,a stable vortex formation adjacent said downstream-facing surface beinginduced thereby, said member having at least one opening in each regionof step change at opposite edge portions of said downstream-facingsurface wherethrough suction is applied.
 2. A combustion device asclaimed in claim 1 including a flow divider extending downstream of saiddownstream-facing surface whereby vortices originating at said stepchange of section at opposite edge portions of said downstream-facingsurface are separated.
 3. A combustion device as claimed in claim 1,said member having two openings therein, each said opening comprising aslot, said slots extending along opposite edges of saiddownstream-facing surface wherethrough suction is applied.
 4. Acombustion device as claimed in claim 1 wherein said member is ofannular form, said member occupying a plane normal to the direction ofgas flow.